(1) Field of the Invention
The present invention relates to a method and to a device for checking the state of health of a turbine engine arranged on a rotary wing aircraft, the aircraft having at least one engine.
(2) Description of Related Art
A rotorcraft is piloted by monitoring numerous instruments on an instrument panel. Most of the instruments are representative of the operation of the power plant of the rotorcraft.
Furthermore, and for physical reasons, there are numerous limits that the pilot must take into account at all times during a flight. These various limits depend in general on the stage of the flight and on external conditions.
Most two-engine rotorcraft presently being manufactured are fitted with two turboshaft engines each having a free turbine for driving rotation of the main rotor that provides propulsion and indeed lift. The driving power is then taken from a low-pressure stage of each free turbine, which stage is mechanically independent of the assembly comprising the compressor and the high-pressure stage of the engine. Each engine free turbine has a speed of rotation lying in the range 20,000 revolutions per minute (rpm) to 50,000 rpm, so a speed-reducing gearbox is needed in order to connect with the main rotor since its speed of rotation lies substantially in the range 200 rpm to 400 rpm. This is the main power transmission gearbox (MGB).
Temperature limits of the engine and torque limits of the MGB serve to define three normal utilization ratings of the engine:
Among known ratings, mention may be made of:
the takeoff rating that associates a maximum takeoff power PMD with a duration of utilization of the order of five to ten minutes;
the maximum continuous rating that associates a maximum continuous power PMC with a duration of utilization that is unlimited; and
a transient rating that associates a transient maximum power PMT with a limited duration of utilization.
There also exist supercontingency ratings for aircraft having at least two engines, these ratings being used when one of the engines fails:
a first contingency rating associates a supercontingency power 30-sec OEI with a duration of about thirty consecutive seconds, this first contingency rating being usable about three times during a flight;
a second contingency rating associating a maximum contingency power 2-min OEI with a duration of utilization of the order of two minutes; and
a third contingency rating associating an intermediate contingency power OEIcont with a duration of utilization extending to the end of a flight after the failure of an engine, for example.
Under such conditions, by calculation or by testing, the engine manufacturer establishes curves for the power available from an engine as a function of altitude and temperature, and does so for each of the above-defined ratings. Likewise, the manufacturer determines the lifetime of the engine and the minimum power it guarantees for each rating, with this minimum guaranteed power corresponding to the power that the engine can still deliver on reaching the end of its lifetime, with such an engine being referred to below as an “aging” engine, for convenience.
In order to verify that the engine is operating correctly, it is appropriate to perform a health check to be sure that the engine presents performance that is greater than or equal to the performance of an aging engine.
In particular, two monitoring parameters are important for checking the performance of an engine.
The engine has a high-pressure turbine arranged upstream from a free turbine, and the first monitoring parameter may be the temperature of the gas at inlet to the high pressure turbine, written TET by the person skilled in the art.
The blades of the high pressure turbine of the engine are subjected to centrifugal force and to the temperature TET. Above a certain level, the material from which the blades are made suffers creep, with the consequence of the blades expanding lengthwise. This causes the blades to come into contact with the casing of the high pressure turbine and thus to be damaged. The temperature TET is thus directly linked with degradation of the engine.
Nevertheless, since the temperature TET is very difficult to measure because of its relatively non-uniform nature, the first monitoring parameter may be the temperature of the gas at the inlet to the free turbine, written T45 by the person skilled in the art. This temperature is a good indicator of the temperature TET and consequently it is representative of the degradation of the engine.
A first monitoring parameter is thus a temperature of a turbine assembly having at least one turbine, which temperature may be the temperature TET of the gas at the inlet to the high pressure turbine, or the temperature T45 of the gas at the inlet to the free turbine.
A second monitoring parameter relates to the power delivered by the engine or to the torque delivered by the engine, where the power and the torque of an engine are closely related. Given that the speed of rotation of the gas generator of the engine, written Ng by the person skilled in the art, is ultimately linked to the power delivered by the engine, the second monitoring parameter used may be this speed of rotation of the gas generator.
Consequently, checking the state of health of the engine consists either:
in measuring the first monitoring parameter and then in verifying that the current power value is greater than or equal to the power value that an aging engine would deliver under the same conditions; or
in measuring the second monitoring parameter and then in verifying that the current power value is greater than or equal to the power value that an aging engine would deliver under the same conditions.
If the result is negative, the manufacturer considers that the health check is unsatisfactory and the engine needs to be overhauled.
The health check needs to be performed rigorously, since if it turns out negative, i.e. if the above-mentioned verifications do not give satisfactory results, then it has a non-negligible impact on potential grounding of the rotorcraft and on the cost of overhauling it.
In this configuration, it is appropriate firstly to ensure that the bad result of a health check is not the consequence of a malfunction of the power plant as contrasted to a malfunction of the engine. Secondly, it may then be necessary to remove the engine so that an operator, e.g. the engine manufacturer, can verify the degradation in performance on a test bench and then replace the defective elements.
It can thus be understood that it is desirable to perform a health check with very great care in order to avoid grounding a rotorcraft without good reason. Unfortunately, it is sometimes difficult to perform a health check under good conditions on a rotorcraft that has a plurality of engines.
For such an aircraft, a first solution consists in performing the health check during a cruising flight. A cruising flight presents the advantage of taking place in a stage of flight that is not disturbing and with an engine that is operating in stabilized manner. Under such circumstances, in order to perform a health check in flight, a pilot places the aircraft in a special flight stage such as level flight at stabilized altitude and speed for several minutes.
Nevertheless, the power developed by the engines during such a flight is well below the reference power levels, i.e. the maximum takeoff power PMD, for example. Unfortunately, a health check is more accurate if the power developed by the engine being checked is close to its reference power.
Furthermore, if the result of a health check performed at low power is unsatisfactory, it is common practice to perform an additional health check at high power. In order to avoid complaints from the passengers of the rotorcraft, who are then disturbed by the vibration generated in the cabin during such a flight, the health check is then often performed during a technical flight dedicated to performing the check, and thus representing considerable expense.
Furthermore, in a two-engine rotorcraft, it is appropriate to ensure that each engine is capable of developing the minimum guaranteed power at the supercontingency ratings. The health check is preferably performed at a rating that is as close as possible in terms of developed power to the supercontingency ratings. As a result, health checks are preferably performed at a power that is close to the maximum takeoff power PMD, which is not compatible with cruising flight.
A second solution then consists in performing the health check during a fast cruising flight, by increasing the power developed by the engines so as to come close to the maximum takeoff power PMD, for example. Nevertheless, although that solution is effective, it gives rise to complaints from the passengers of the rotorcraft who are disturbed by the vibration generated in the cabin as a result of the conditions of the flight.
In order to remedy that, the proprietor of a two-engine rotorcraft may perform a specific technical flight dedicated to carrying out the health check in the absence of passengers. The impact of such a flight on the maintenance costs of the rotorcraft is not negligible, insofar as the manufacturer of the engine generally sets the periodicity for health checks in the range 25 hours (h) to 100 h. Thus, each technical flight takes the place of a paid-for flight, thereby giving rise to a considerable cost for the proprietor of the rotorcraft.
A third solution consists in increasing the power that is developed, but only on the engine being checked. Although attractive, that solution presents drawbacks.
Since the rotorcraft has two engines, that means that the engines are no longer aligned in terms of power. Consequently, modern engine computers detect a loss of power. Under such conditions, a red warning is activated by the computers to inform the pilot that it is essential to land the aircraft. Furthermore, such detection leads to the supercontingency ratings being prepared.
Document FR 2 899 640 describes a method of performing a health check of at least a first engine of a rotorcraft, the rotorcraft having first and second engines presenting respective first and second current values for a monitoring parameter prior to the health check, and respective first and second real final values for the monitoring parameter during said health check. The following steps are performed in succession:
a) determining the first real final value of said monitoring parameter that said first engine is to reach in order to perform said health check accurately;
b) assuming that said second real final value of said monitoring parameter of said second engine is equal to said second current value of said second engine;
c) determining the difference between said first real final value and said real second final value;
d) if said difference is greater than a predetermined threshold, readjusting said second real final value so that the difference between said first real final value and said second real final value is less than said predetermined threshold during the health check; and
e) controlling said first engine so that said first current value before said health check reaches said first real final value during said health check and controlling said second engine so that said second current value before said health check reaches said second real final value.
In another technique, a health check may also be performed on the ground in a configuration close to takeoff. For example, on a two-engine aircraft, one engine may be idling while the other engine develops power close to the intermediate contingency power OEIcont.
Although advantageous, a health check performed on the ground can be inaccurate because of the ground effect to which the aircraft is subjected.
In addition, it can be understood that a health check is performed by comparing the performance of the engine under test with minimum performance levels, such as the test bench performance levels declared by the manufacturer. The health check makes it possible to determine a margin for a monitoring parameter of an engine compared with a limit value for the monitoring parameter.
However, depending on the engine, health checks are not always performed using the same procedure, each manufacturer establishing its own procedure.
Furthermore, calculating the operating margins of an engine depends on the rating implemented, on atmospheric conditions, on stabilization conditions of the engine, and on the effects of mounting the engine in an aircraft, also known as installation losses.
Installation losses give rise to losses of power, e.g. due to head losses in the air inlets of the engines or to pressures being distorted, or even to the exhaust nozzles. Furthermore, installation losses also come from power being taken off from the engine by accessories presenting operation that depends on the altitude of the aircraft and on the outside temperature, in particular.
Such installation losses lie behind differences between the values of monitoring parameters when an engine is arranged on a test bench and when the same engine is mounted on the rotorcraft. Installation losses thus have an influence on the comparison between the results of the health check and the results obtained on a test bench, e.g. using an aging engine.
The margins of an engine may thus be different from one health check to another for reasons that are independent of the state of health of the engine. Under such circumstances, it can be difficult to track trends in the health of the engine.
The following documents are also known: FR 2 902 407 and U.S. Pat. No. 7,487,029.